Metalloproteins are a class of proteins having a metal ion complexed with the protein molecule at the protein's metal-binding site. The metal ion provides features to a metalloprotein such as facilitating electron transfer, oxidation, or reduction reactions and the like.
The stereochemistry of metal ion complex structure in association with protein has been extensively characterized. Studies of known metalloproteins have resulted in characterization of many metal ions that participate in metal protein complexes, which helps to identify the nature of the metal-protein complex. Three dimensional crystal structures of metalloproteins are available and provide further insight into the nature and structure of the metalloprotein complexes. See, for example, the structures identified for naturally occurring metalloproteins, namely thermolysin, superoxide dismutase and carbonic anhydrase. Holmes et al., J. Mol. Biol., 160:623-639 (1982); Tainer et al., Nature, 306:284-287 (1983); and Eriksson et al., Proteins: Struct. Funct. Gen., 4:274-282 (1988).
Interest in producing proteins with increased structural stability has motivated investigators to introduce a variety of conformational constraints in the polypeptide structure of a protein. See, for example, Kemp et al., Tetrahedron Lett., 9:4931 (1988); Arrhenius et al., Protein Structure and Design II: UCLA Symp. Mol. Cell. Biol. New. Ser., 9:453 (1987); Arrhenius et al., Vaccines, p. 17 (1989); Felix et al., Int. J. Pept. Prot. Res., 32:441 (1988). Conformational constraint approaches have been motivated because formation of secondary structure in disordered polypeptides requires a first nucleation event, with the energetically unfavorable formation of the first turn of the secondary structure being rate limiting. See for example, Zim et al., J. Chem. Phys., 31:526 (1959); Sueki et al., Macromolecules, 17:1948 (1984); Vasquez et al., Biopolymers, 26:351 (1987); Schwartz et al., Angew. Chem. Int. Ed. Engl., 11:568 (1972); and Gruenewald et al., Biophys. Chem., 9:137 (1979). Synthesis of polypeptides incorporating conformational constraints require considerable synthetic effort and have been difficult to apply to larger polypeptides and proteins.
Although stabilization of secondary structure has been reported to be facilitated by metal ions in association with polypeptides, structural data indicates that those stabilized metallocomplexes typically involve three or more amino acid residue side chains as donors for ligands to the metallocomplex.